This invention relates to data transmission systems and more particularly to a deterministic (i.e. delays to access a channel are limited or bounded) multiaccess method for accessing a control channel of a decentralized mobile radio system before a voice channel is permitted to be accessed, wherein the system includes a base radio station, a repeater station and one or more mobile or portable radios among which data may be transferred. Each base station, repeater station and radio comprises a transmitter and a receiver or a transceiver. Several known multi-access methods are described in "Multiaccess Protocols in Packet Communication Systems" - F. A. Tobagi, IEEE Transactions on Communications, Vol. COM-28, No. 4, p. 468 (1980) [hereinafter Tobagi].
In some commmunication systems, such as TELENET network of Telenet Corporation and others using the X.25 protocol of International Consultative Committee for Telegraphy and Telephony (CCITT), a calling station and a called station perform a handshake routine (i.e. transmission of a call request and a call accept packet, respectively) to determine if the called station is free to accept an incoming call before a virtual circuit can be set up for data transmission between the calling and called station. It is to be understood that the terms data channel, talk channel, and voice channel as used herein are interchangeable and indicate that either voice (encoded voice) and/or data may be transmitted over the channel.
In mobile radio systems random multi-access protocols, such as ALOHA (see Tobagi, supra, p. 471), have been used. However, only about 18 percent of maximum channel utilization is possible for pure ALOHA. Slotted ALOHA (see Tobagi, supra, p. 471) allows only about 36 percent of maximum channel utilization and requires synchronization of all transmitters. If the load, i.e. number of stations attempting to access a control channel, exceeds the allowable utilization, the control channel throughput decreases and may drop to zero, even if a voice channel is idle.
Another possible protocol is a token passing scheme (for one type see Tobagi, supra, p. 483), i.e. permitting a station to transmit only during the interval it controls the token. However, a token passing scheme is very difficult to implement for a decentralized system. Among the problems of using a token passing scheme in a mobile radio environment are the following: (1) Only a portion of all stations, i.e. mobile and base, assigned to a logical ring may be active (i.e., turned on) and polling to determine active stations takes too long (a logical ring comprises physical units, such as stations, having a logical number assigned to each unit. The units are operated in a ring configuration (which may not be the same as physical arrangement or interconnection of the units) based on their assigned logical number in order to provide a modular type of operation.); (2) there is no centralized control or monitor to administer the system, i.e. find lost tokens, determine which stations are active, provide an up-to-date directory, e.g. in which cell a mobile station is presently located. (A cell is a geographical area within which a logical ring exists and stations physically located therein can hear each other's transmission via a repeater, i.e., the area over which reception from a repeater is possible); (3) it is not possible for all active stations to receive all packet transmissions due to propagation distance limitations; (4) a specific predetermined control channel frequency is typically assigned to each mobile and base receiver to monitor whereas each mobile and base transmitter can typically transmit at all control channel frequencies, and thus it is not possible to predict the level of usage of each control channel at any instant; (5) if a called station is not active, so that a call accept or call reject (busy) packet is not received by the calling station, then the calling station typically assumes that a collision with its call request packet has occurred and the calling station subsequently attempts to regain access to the control channel, resulting in undesirable increased control channel load.
Consider a hypothetical mobile radio communication system employing a carrier sense multiple access (CSMA) (see Tobaji, supra, page 471) protocol over a control channel, before access to a voice channel is permitted. Assume that at an instant during a peak hour all voice channels are busy and there are N stations, which are not transmitting but desire to transmit a control packet, waiting in a control packet queue. Also assume that the voice channel queue (stations desiring access to a voice channe1) is growing at the rate of m stations per second. If T seconds later a voice channel becomes available, there will be M=N+m.times.T stations contending for a control channel in order to gain access to the free voice channel, either immediately or t seconds later, where t is a fixed number used by all busied out stations. Thus all M packets will collide with each other. Moreover, if q retransmissions are permitted after being unsuccessful in securing a voice channel, then the number of control packets in the control packet queue may eventually reach q.times.M packets, while the voice channel queue is still growing at the rate of m stations per second. The system would block, i.e. free control channels could not be accessed, for an indeterminate period of time, despite one or more voice channels being free.
One method for avoiding some of the problems experienced by the system described in the previous paragraph is to use a p--Persistant Carrier Sense Multiple Access protocol (see Tobagi, supra, p. 472). In accordance with this protocol, when a voice channel becomes available, M stations transmit with a probability of p and do not transmit with a probability of (1-p). In order to have one successful transmission, i.e. one station gain access to a control channel, the mean of the binomial probability distribution must be equal to one, i.e. p=1/M. However, it is very difficult to accurately estimate the value of M in a complex decentralized communication system since there is by definition no centralized authority to determine who was involved in a collision and which stations of all possible stations are included in M at any instant, i.e. which stations are trying to transmit.
A more advanced protocol, carrier sense multiple access with collision detection (CSMA/CD) (see Tobagi, supra, p. 473), requires that a station back-off, i.e. wait a predetermined interval after experiencing an unsuccessful attempt (e.g. collision with another packet, failure to receive a call accept or call reject packet) to access a control channel, before again trying to gain access to the control channel. This back-off procedure penalizes the unsuccessful station since stations newly desiring to gain access to the control channel may be successful during the back-off interval of the unsuccessful station. It would be desirable to provide priority status to an unsuccessful station such that all unsuccessful stations are permitted access to the control channel before any station not having experienced an unsuccessful attempt is permitted to try to gain access to the control channel.
Accordingly, it is an object of the present invention to provide a deterministic control channel accessing scheme for a mobile radio environment allowing up to about 80 percent control channel utilization.
Another object is to provide priority access to a control channel for emergency numbers such as police, fire department, ambulance, etc.
Still another object is to provide priority access to a control channel for stations having had an unsuccessful attempt to gain access to the control channel.
Yet another object is to minimize the number of collisions which result from inability to detect all transmissions by all stations.